In mixture-compressing internal combustion engines having external ignition and internal mixture formation, stratified charge operation requires a “mixture cloud” in the region of the spark plug, which must have a particular fuel-air ratio in the ignitable range. Especially in partial load operation and during idling, this has a considerable effect on fuel consumption since the entire combustion chamber volume is not filled with an enriched, ignitable mixture.
Basically, three methods are known for producing such a mixture cloud: the wall-directed, the air-directed and the jet-directed fuel-injection system.
In the wall-directed fuel-injection system, the fuel is carried along by an air vortex which lies up against the wall of the cylinder and in most cases is deflected at the specially designed piston. For this purpose, a fuel injector, which is positioned at the side of the intake valve toward the cylinder wall, injects the fuel into a vortex near the wall surface of the cylinder and/or onto the piston surface. The vortex causes the fuel to be carried along to the spark plug in the center of the combustion chamber, a mixture being formed at the same time.
A wall-directed fuel-injection system is known from European Published Patent Application No. 0 519 275, for instance. A spark plug is positioned in a center section of an inner wall of a cylinder head, and a fuel-injection device is located at a circumferential section of the inner wall of the cylinder head. A piston has an indented section beneath the spark plug, onto whose surface the fuel is injected at a slant, the fuel being deflected at the edge of the indented surface. A vortex carries along the fuel in the direction of the spark plug.
Disadvantageous in this related art is that the turbulence-producing measures, such as special designs of the combustion-chamber geometry, especially the piston form designs, can only be optimized for a specific speed range. Especially in the area of partial load and idling, i.e. when small fuel-injection quantities and low filling degrees and/or low speed are involved; the required vortex formations can only be achieved by throttling. This, however, results in higher fuel consumption. Moreover, it is disadvantageous that the wall-directed fuel-injection system requires a more costly piston design with a deflection edge.
From German Patent No. 38 08 635, a fuel injection device for the direct injection of fuel into the cylinder of a mixture-compressing internal combustion engine is known in the form of an air-directed fuel-injection system. The fuel injection device includes a fuel injector, which is positioned in the cylinder wall at a certain distance from the cylinder head and across from the discharge orifice; it has an outlet opening, the jet axis of the fuel injector being directed to the region around the spark plug positioned in the cylinder head. The fuel injector includes a magnet-activated valve needle provided with helical swirl grooves to generate a swirl flow of the injection jet. The entire cross-sectional area of the swirl grooves is smaller than the cross-sectional surface of the outlet opening by at least half, the fuel injector being positioned above a flushing orifice and its jet axis pointing to the ignition point located in the center of the cylinder head. Owing to the air stream, the injected fuel is carried along in the direction of the spark plug and conveyed thereto without touching the cylinder wall.
In air-directed fuel-injection systems as well, the mixture conveyance to the spark plug is less than successful in idling operation and in lower partial-load operation, due to the aforementioned reasons; in mean partial-load operation it is partly only possible with unjustifiably low production variances of the high-pressure fuel injectors utilized and/or the flow guidance through the intake manifold. The faulty reproducibility primarily manifests itself in increased emissions of unburned hydrocarbons as a result of individual combustion misses.
From German Published Patent Application No. 198 04 463, a jet-directed fuel injection system for mixture-compressing internal combustion engines having external ignition is known, which includes at least one fuel injector injecting the fuel into a combustion chamber having a piston/cylinder design, and which is provided with a spark plug projecting into the combustion chamber. The nozzle body of the fuel injector is located in the vicinity of the spark plug, which is in the center of the combustion chamber, and it is provided with at least one row of injection orifices distributed over the circumference of the nozzle body. By selectively injecting fuel via the injection orifices, a jet-controlled combustion method is realized by a mixture cloud being formed, at least one jet being aimed in the direction of the spark plug. Additional jets ensure that an at least approximately continuous or cohesive mixture cloud is formed.
A method of forming an ignitable fuel/air mixture is known from German Patent No. 196 42 653. An ignitable fuel/air mixture can be formed in the cylinders of internal combustion engines having direct injection in that an injector injects fuel into each combustion chamber bounded by a piston upon release of a nozzle orifice in response to a valve member lifting off from a valve seat which encircles the nozzle orifice. To produce an internal mixture formation that is optimized with respect to consumption and emissions, in every operating point of the entire characteristics map, under all operating conditions of the internal combustion engine, particularly in stratified operation, the opening stroke of the valve member and the injection time are variably adjustable.
Disadvantageous in jet-directed fuel-injection systems is that currently used cylinder heads having a spark plug centrally positioned in the cylinder axis in the cylinder head, can only be used when substantially modified, since the additional installation space for the fuel injector in the vicinity of the spark plug requires a disadvantageous moving of the, in most cases, two intake valves.
While the currently used production tools and cylinder-head designs may in fact be compatible when the fuel injector is positioned in the cylinder wall on the side of the intake valves, the afore-described disadvantages of the air-directed and the wall-directed fuel-injection systems still occur.